DESCRIPTION

Counting semaphores provide a mechanism for synchronizing access to a
pool of resources. Unlike mutexes, semaphores do not have the concept of
an owner, so they can also be useful in situations where one thread needs
to acquire a resource, and another thread needs to release it. Each
semaphore has an integer value associated with it. Posting
(incrementing) always succeeds, but waiting (decrementing) can only
successfully complete if the resulting value of the semaphore is greater
than or equal to zero.
Semaphores should not be used where mutexes and condition variables will
suffice. Semaphores are a more complex synchronization mechanism than
mutexes and condition variables, and are not as efficient.
Semaphores are created with sema_init(), where sema is a pointer to space
for a structsema, value is the initial value of the semaphore, and
description is a pointer to a null-terminated character string that
describes the semaphore. Semaphores are destroyed with sema_destroy().
A semaphore is posted (incremented) with sema_post(). A semaphore is
waited on (decremented) with sema_wait(), sema_timedwait(), or
sema_trywait(). The timo argument to sema_timedwait() specifies the
minimum time in ticks to wait before returning with failure.
sema_value() is used to read the current value of the semaphore.

RETURNVALUES

The sema_value() function returns the current value of the semaphore.
If decrementing the semaphore would result in its value being negative,
sema_trywait() returns 0 to indicate failure. Otherwise, a non-zero
value is returned to indicate success.
The sema_timedwait() function returns 0 if waiting on the semaphore
succeeded; otherwise a non-zero error code is returned.